Balanced frequency modulation system



March i3, 1945. A,` BADMAIEFF 2,371,373

BALANCED FREQUENCY MODULATION SYSTEM Filed June 12, 1945- 4 sheets-sheet1 1N V EN TOR. A2 vr/L5 5404/4/59? BY I A TTOR NE Y March 13, 194. A.BADMAIEFF 2,373,373

, BALANCED FREQUENCY MODULATION SYSTEM Filed June 12, 1943 4Sheets-Sheet 3 IN VEN TOR. L E z/J 40AM/5F@ A TTOR NF V Max-ch 13, i945.A. BADMAIEFF 2,371,373

BALANCED FREQUENCY MODULATION SYSTEM Filed June 12, 194s 4 sheets-sheet4 2 3 4567.891 2 3 4567894 Z 3 ooo wooo zooao Z 3 4 5 6 7 9 20 foo/ifauEA/cy /N cru es Peyssc ATTORNEY Patented Mar.' 13, 1945 UNITEDSTATES PATENT OFFICE BALAN CED FREQUENCY MODULATION SYSTEM AlexisBadmaie', Indianapolis, Ind., assignor to Radio Corporation of America,al corporation of Delaware Application June 12, 1943, Serial No. 490,614

14 Claims.

criminator are usually a short distance apart, or

even combined in one unit. Such systems have been applied to soundreproducing devices, and to measuring devices. In such prior systems anoscillator is provided with a frequency changing transducer, and thetransducer is varied in response to a vibratory member thereby tofrequency modulate the oscillator mean frequency.

The frequency modulated energy is then fed to a discriminator circuit,tuned to a slightly different frequency so that the oscillator meanfrequency falls on one side of the discriminators resonance curve. Thediscriminator converts the frequencyvariable wave energy intolamplitude-variable wave energy. Subsequent rectification derives theoriginal modulating frequencies from the amplitude-variable wave energy.

I have invented a method of greatly increasing the voltage output ofsuch a system. The method is simple, and employs a minimum of elements.The method essentially comprises the concurrent variation, from onemodulating source, of the oscillator frequency and the discriminatorresonance peak in opposite directions.

From a generic viewpoint, it may be stated that it is one of theprincipal objects of my invention to provide a method wherein a properlyconstructed balanced transducer is employed to vary the oscillator anddiscrimlnator circuit tuning in opposite directions, or in oppositephase relation, simultaneously.

It is an important object of my present invention to provide a frequencymodulated ("FM for brevity hereinafter) oscillator feeding into adiscriminator whose resonance peak is subjected to concurrent FM action,the two frequency modulations being 180 degrees out' of phase wherebythe discriminator voltage output will be twice as great as would be thecase if only one of said FM actions had been employed.

The two forms of FM may be provided simply in anoscillator-discriminator network by employing a balanced transducer,more specifically a pushpull condenser, as the modulating device. Oneside of the condenser can then be connected across Vthe oscillator coil,while the other side is connected across the discriminator coil. Hence,if

the grounded center plate is moved either way, the direction of thefrequency changes of the oscillator and discriminator circuits will beopposite.

Another important object of this invention is, therefore, to provide anovel FM circuit utilizing a symmetrical modulating device therebyproviding cancellation of even harmonics, greatly reducing distortion,and doubling the output voltage from the discriminator.

The present invention has particular utility in an FM system forreproducing recorded sound waves. IOne such FM system has beendisclosed, for example, by C. M. Sinnett in application Serial No.459,375, led September 23, 1942. The particular Sinnett system is asingle-ended frequency modulation network employing a capacity pickupdevice of the ribbon type for the reproduction of sound recordings. Inthe Sinnett system the frequency modulation network consists of anelectron discharge tube which has one section thereof functioning as ahigh frequency oscillator of constant amplitude whose frequency isdeviated by variations in the capacity of the pickup. In the platecircuit of the tube there are developed frequency modulated oscillationswhich are subsequently converted into audio modulation signals,

corresponding to the originally recorded audio signals, by a frequencydiscrimination process.

By applying the generic method of my invention to the particularySinnett system, or in general to any FM system of record reproduction,various advantages are secured. The advantages in my system result inreduction of the mass of the vibratory pickup system; reduction of evenharmonics; reduction of wear on the record surface; and an increase inthe amplitude of the audio output voltage. I have found that theseadvantages may be realized by employing an additional capacitativevariation across the discriminator circuit. 'I'he additional capacityvariation is effected by the same capacitative vibratory system which isutilized across the oscillator tank circuit. Furthermore, the dualcapacity-variations may be so correlated that the changes in theoscillatory frequency and the discrlminator circuit resonance occurdegrees out of phase, whereby a push-pull frequency deviation eect issecured. Cancellation of even harmonics, reductioof inter-modulation andhigher audio voltage ou ut are immediately realized by such a system.The higher audio voltageoutput can be employed to advantage, because thesize of the pickup condenser plates can be reduced.

Accordingly, it may be stated that it is another object of my presentinvention to provide an FM "-network for with-asimple speciallyfdesignedf capacity pickup device,l which is'adapted to rev produce lateral orvertically cutrecords, and there being provided "a frequency modulated`signal whose frequency deviation is the cumulative. effect of oscillatorand discriminator frequency variation in push-pull relation whereby thevarious aforementionedadvantages may be secured.

In a pickup it is most important to keep the mass of the vibratorysystem as low as possible. 'I'his raises the frequency of the peak andreduces the mechanical impedance, thereby permittinga. reduction ofneedle pressure and the accompanying reduction of surface noise.

Accordingly, it may be stated that it is another important object of myinvention to provide al capacity pickup device wherein the stylussupporting wire acts as the center plate of a pushpull condenser workinginto a balanced, or pushpull, FM oscillator-discriminator networkwhereby the mass of the pickup is reduced to a. minimum, since nothingelse` would be mechanically coupled to the stylus except its ownsupport; and the supporting wire being small in diameter so that noextra mass would be added in transforming the mechanical vibrations toelectrical frequencies. l Still otherobjects of my invention are to improve generally the simplicity and emciency of record reproducingequipment of the FM type, and more especiallyv to provide a high delity,low cost and low distortion record reproducer system which is easilymanufactured .and readily assembled. s f

The novel features which I believe to be characteristic of my inventionare `set forth with particularity in the appended claims; the inven tionitself, however, as to both its'organization and method of operationwill best be understood by reference to the following description, takenl a cathode tap on the grid coil may be-employed.

in connection with the drawing, in which I have indicateddiagrammaticallv several circuit organizations whereby my invention maybe carried into effect. Y

In the drawings:

Fig. 1 shows a generalized circuit embodying the invention,

Figs. la and lb schematically show modifications,

Fig. 2 graphically illustrates the FM action at the oscillator circuit,f

What is essential is that tube l be provided with v anoscillation-producing circuit to produce highA frequency oscillations ofa certain mean frequency. g f

The oscillatory energy isv applied to the reso nant input circuit 5 of asecond tube 4. vThe circuit 5 is slightly off-resonance relative tothemean frequency of the oscillator. VThat is to say, the oscillator may beoperated at a frequency located on somewhat either side of the resonancepeak of circuit 5. This provides the Well known discriminator action atcircuit 5. Tube 4 may be operated as a grid leak detector, or as a platecircuit-detector. For purely illustrative purposes, the leaky gridresistor 6 is shown in the grid circuit of tube 4. There is providedgrid circuit rectification of any amplitude-variable energy developedacross the discriminator circuit 5, The detector output may be utilizedin any desired manner.

In the past it has been proposed to vary the frequency of the tunedcircuit 2 by any form of periodically-variable y reactance device. Forexample, a variable condenser shunted across circuit 2 has been employedto provide a variation in frequencyi. e., a frequency modulation of theoscillatory energy. This may be accomplished, for instance, by having avibratory member respond to a modulating source and vary the condensercapacity. The frequency-variable oscillations are applied to theoff-tune circuit 5 l This aforedescribed action exists regardless ofWhether the modulating condenser is part of a u microphone, `a stylus, acutter, or a measuring Fig. 3 graphically illustrates the FM action atthe discriminator, v

Fig. 4 illustrates the conjoint FM action at the oscillator anddiscriminator circuits,

Fig. 5 shows the circuit details of an FM reproducer of phonographrecordings 'embodying the invention,

Fig. 5a shows the circuit diagram of a preferred embodiment of theinvention,

Fig. 6 shows an end view of the pickup;

Fig. 7 vshows a bottom view of the pickup;

Fig. 8 shows a detail of the stylus support wire;

Fig, 9 is a section along line 9--8 of Fig. 8;

Fig. 10 shows acoustic response curves of the present invention. i

Referring now to the accompanying drawings, wherein like referencecharacters in the diiferent iigures denote similar circuit elements,there is shown in Fig. 1 a generalized circuit arrangement embodying mypresent invention. The numeral l designates any type of electrondischarge tube whose grid circuit 2 is tuned to a predeterminedoscillatory frequency. The tube has a tickler coil 3 in the platecircuit to produce oscilinstrument. The general process of translatingla. modulating signal into an FM oscillation, conversioninto anamplitude modulated (AM for brevity) oscillation, and re-translationinto the original modulating signal is common to all the applications.Considering the operation more specifically, and referring to Fig. 2,there can be graphically depicted the relation between detector outputvoltage and FM oscillatory energy applied to the discriminator. In Fig.2 the full line curve represents the discriminator resonance curve whosepeak frequency is higher than the mean, or center, frequency Fc of theFM oscillations. By virtue of the slope of the side of the resonancecurve, the frequency swings of the oscillations are converted intoVariations in am plitude of the detector output voltage. put voltageamplitude will depend on the modulation of the FM energy.

Assume, now, that the variable condenser across the oscillator circuit 2has been replaced by a fixed condenser of a value equal to the formercondenser when not modulated, so as to produce a frequency equal to theformer center frequency This discrimination action is very` The Out-`2,a71,s7s again graphically represented in Fig. 3, where the resonantpeak of the discriminator will move back and forth along the horizontalfrequency axis; andy the oscillator frequency remains xed. In the courseof these illustrations, it is assumed that the L vto C ratios in boththe oscillator circuit 2 and the discriminator circuit 5 are very nearlythe same.

If we now combine both modulation actions, respectively depicted inFigs. 2 and 3, in one system, and provide modulation from a singlemodulating source there is secured a balanced, or pushpull, FM action.In Fig. 1 I have shown a simple method of securing such balanced FMaction. Al double condenser is provided with its center plate 1 mobileand grounded. One xed plate 8 is conf nected to the high potential sideol oscillator circuit 2, while the opposite xed plate 9 is connected tothe high potential side of discriminator circuit 5. Motion of plate 'Iwith respect to plates 8 and 9 will now result in a shifting of theoscillator frequency in the opposite direction from the discriminatorresonance peak frequency.

Since the two frequency shifts, or modulations, are always 180 degreesout of phase, the voltage output from the detector will be twice asgreat as would be the case if only one of the frequency shifts had beenprovided. This is clearly depicted in Fig. 4, wherein there is shown thecombination of the FM actions of Figs. 2 and 3. It will be noted thatthe detector output voltage in the case of Fig. 4 is double that ofeither Fig. 2 or Fig. 3. When the oscillator frequency is moved downalong the frequency axis, and at the same time the discriminatorresonance peak is moved up, the relative shift along the frequency axisis doubled thereby producing a double amplitude in the voltage output ofthe detector.

The frequency changing device', or balanced transducer, of Fig. 1 neednot be of the capacitative type. In Fig. la I have shown how a variableinductance can be used for the Afrequency modulation process. Morespecifically, in Fig. 1a the modulating device is a shorted loop whichis adapted to move between the oscillator and discriminator tunedcircuits. The shorted loop acts as a modulator and inductive shield. Inthis way, the respective frequencies of the oscillator and discriminatorare varied concurrently and in opposite senses. It will be understoodthat the shorted loop will be provided with a pivoted stylus, and thearrows adjacent the stylus and loop indicate the directions of motion ofthese elements.

In Fig. lb there is shown still another manner of concurrently Varyingthe oscillator and cliscriminator frequencies in opposite senses. Inthis case the iron cores of the oscillator and discriminator coils areactuated by a pivoted stylus to adjust the respective frequencies. Thoseskilled in the art are fully aware of the manner of constructing anelectron discharge tube circuit so that it can function as an electronicreactance device. If an electronic transducer is required, a pair ofelectronic reactance tubes can be connected across the respectiveoscillator and discriminator tuned circuits for simulating the effectsshown in Figs. 1, la or 1b.

It will, therefore, be'appreciated that my invention contemplates ageneric method of varying the tuning of an oscillator and discriminatorby a balanced transducer of appropriate construction, the tuningVariation being concurrent and in opposite phase. By the term balancedin this application'is meant any concurrent variation of two effects inopposite phase relation.

There. are man'y advantages which result from the utilization of thisbalanced FM action when `applied to a balanced capacity pickup device.In

the first Place, it is only necessary to employ a simple push-pullcondenser. One side of the latter is connected across the oscillatorcoil, while the other `side of the condenser is connected acrossthe-discriminator circuit coil. In that case if the center groundedplate is moved either way, the directid of the frequency changes'of theoscillator an discriminator circuits will be opposite. The advari age ofsuch a system is primarily the cancellatio of even harmonics. When onecondenser plate is moved away from the other, further movement'has lesseffect on its capacity because the capacity varies inversely as thedistance of separation. This results in a non-linear relation betweencapacity and displacement with resulting distortion. By combiningvoltages from two similar non-linear elements in a balanced system thereis secured a great reduction in distortion with practical elimination ofthe rectifi- .cation and even harmonic components of the distortion.

Another advantage of the balanced FM action is that the amplitude of theoutput voltage at the detector is doubled for a given amount of capacitychange. In other Words, for a given amount of mechanical deflection ofthe grounded center condenser plate there is secured double the detectoroutput voltage amplitude. 4If the output voltage amplitude is kept thesame, the capacity increv ment can be half as much when using the FMbalanced action. 'I'his has a practical result insofar as the mechanicalconstruction of the modulating condenser is concerned. The condenserplates can be made smaller, and, in turn, it is not necessary to attachany plate to the wire which carries the stylus, assuming that the systemis to be used in the reproduction of phonograph records. In the lattercase there is not only secured considerable mechanical simplification,but the' mass of the vibratory system is reduced thereby lowering itsmechanical impedance.

While in Fig. 1 I have shown a simple form of oscillator followed by jadiscriminator-detector circuit, in Fig. 5 there is shown my genericmethod applied to the particular type of single-ended FM circuitreferred to hereinbefore as the Sinnett system. The latter employsan'electron coupled v form of oscillator-discriminator circuit. It isagain emphasized that my invention may be applied to any other type ofFM record reproducer. Since the particular circuit of Fig. 5 in itssingleended formhas been disclosed and claimed in the aforementionedSinnett application, it is not believed necessary to describe thecircuit details specifically, reference being made to Fig. 1 of theSinnett application for a detailed disclosure of the circuit.

It is believed sufficient for the purposes of this application toexplain generally the nature of the circuit, and the specific manner inwhich my present invention is advantageously applied thereto. The tubeI0 in Fig. 5 is shown as of the pentode type. Its cathode is connectedto a low potential tap on the oscillator coil II. The coil II ispreferably of the variable inductance type, and hasan adjustableA coreof comminuted iron. The control grid I2, which functions as theoscillator section grid, is connected through the direct currentblocking condenser I3 to the high potential side of coil II and to onefixed plate I4 of ond fixed plate of the balanced condenser pickup isdesignated by numeral I3. Q

The mobile element IB of the pickup device is connected by lead I'I tothe negative terminal, which may be ground, of the direct currentenergizing source. The positive terminal of the source may be at +300volts. The positive terminal is connected through series resistors Iland I 3 to the electrode 20 of tube Ill. Electrode 20 functions as theoscillator anode of the tube. The output plate 2| has one connectionthrough the direct current blocking condenser 22 to the fixed plate IIof the balanced capacity pickup device. A second conductor connectsplate 2| to the high potential end of the discriminator coil 23 inaccordance with my invention.- The coil 23 is, also, a variableinductance having a core of comminuted iron, and the low potential endof coil 23 is connected to the junction of resistors I9 and I8. Thethird grid of tube lll may be connected back to lead II, and properbypass asf/11,1178

discriminator circuit. Hence, the subsequent detector, or rectier, willhave an input circuit whose operating point is along a linear portio "ofthe discriminator resonance curve. 1 Y Rectification of the resulting AMoscillatory energyis'effected bya simple diode circuit.' If

the 6R? type. 'The numeral 30 designates such-a tube. The lattercomprises a cathode 3|,a plate 32, a control grid 33 and a diodeauxiliary anode 3|. Thel anode 34 and cathode 3i function as therectifier diode. The anode 34 isconnected to its `cathode 3| through apath comprising coil 35 and resistor 3B. Coil 35 is a filter choke toprecondensers areemployed for the energizing leads feeding the anode andplate 2 I.

The cathode, grid I2 and anode electrode 2li of tube I0 provide thelocal voscillator section of the system. Coil II is resonated to themean, or center, frequency Fc. This mean frequency is to be deviated inaccordance with the amplitude of the modulation frequencies. The meanfrequency of the oscillator may be chosen from a range of 30 to 60megacycles (ma). The deviation range of the mean frequency is dependenton the capacity change and the mean frequency employed. A deviation upto about 1% of the mean frequency could be employed, but it should benoted that this total deviation is the sum of the oscillator deviationand the discriminator resonance peak deviation. The choice of the meanfrequency of the oscillator is dependent on the sensitivity that isrequired from the pickup device. Since the capacity change, due tovibration, is of constant range as compared to the amount of deflection,then the higher the frequency the more this given capacity change willproduce a frequency swing. Therefore, the mean frequency should be ashigh as possible, and yet not too high to be unstable, My invention isentirely independent of specific frequencies disclosed herein.

Of course, the balanced pickup device is schematically represented inFig. 5. However, it will be understood that in accordance with mygeneric method the condenser I4-i5 is connected across the oscillatorcoil II, While the condenser IE-IG is shunted across the discriminatorcoil 23. The discriminator coil should` be slightly off-tune from theoscillator mean frequency, either above or below. For example, theoscillator mean frequency, as shown in Fig. 4, may be below the olis-vcriminator peak resonance frequency. The amount of off-tune vdepends onthe"Q of the discriminator resonance, which is the slope of the twosides of the peak. The oscillator mean frequency should be tuned to themiddle portion of a linear part of the discriminator curve side. Themaximum deviation could be of the order of 120 kilocycles (kc.) oneither side of Fc. The generic expression angle modulation is usedherein to designate that the modulation of the high frequencyoscillations may either be frequency, phase or hybrids thereof.Preferably,it is desirable to adjust the iron core of coil 23 so thatthe peak frequency of the discriminator circuit is of a value such thatthe mean oscillator frequency Fc falls at approximately 70% of theresponse on one side of the resonance curve characteristic of the ventthe radio frequency components from reaching the grid 33 of tube 30. Themodulation voltage is developed across lresistor 36, and is ap-` pliedto the control grid 33 through the audio coupling condenser 31. Thecontrol grid 33 is connected back to the lower end of the cathode biasresistor 40, properly bypassed for carrier currents, through the returnresistor 4I.

The plate 32 of tube 3D is connected through the output resistor 50'tothe positive terminal of the voltage supplyvsource. The condenser 5Icouples the diode anode 34 to the high potential side of thediscriminator coil 23. The audio modulation voltageI developed acrossresistor 36, is aml pliiied in the triode section of tube 30. Themodulation voltagedeveloped across the plate load resistor 50 istransmitted by the audio coupling condenser 50 to any utilizationcircuit. For example, when reproducing a phonograph record an audioreproducer may be coupled to condenser 50', or one or more audioampliers may be interposed between the condenser '50' and the audioreproducer.

The system shown in Fig. 5 embodying my generic method utilizescommercially available tubes, and possesses high fidelity. Furthereconomy of tubes may be realized, and other advantages secured, byemploying my preferred circuit of Fig. 5a. In this circuit the circuitcomponents corresponding to similar components of Fig. 5 aredistinguished by the prime designation. The cost of manufacture of asystem as shown in Fig. 5a would be many times less than that of theconventional record reproducers` able to approach the high fidelity ofreproduction secured by my present method. Considering the system ofFig. 5a, the tube I0' functions to provide the electron coupledoscillator, discriminator and detector. The tube may be one of the GSF?type, or any other type of tube which will provide a. cathode,oscillator grid I2', oscillator anode 20', suppressor 20", plate 2| andan Yauxiliary anode 34'. The auxiliary anode 34' cooperates with thecathode y of tube I0' to provide a diode rectification device similar todiode 34-3I of Fig. 5. The suppressor 20" acts as an electrostaticshield between the plate 2| and the other elements.

The cathode of tube Ill' in Fig. 5a is grounded, instead of floating asin Fig. 5. Furthermore, the screen grid 20 functions as an oscillatorplate, and feeds into the lower part of the coil II'. By way ofillustration only, the upper section of coil i I' may have 5 turns whilethe lower section has 2 turns. The discriminator coil 23' may have 5turns. These are purely illustrative values, and do not affect the scopeof my invention. Condenser C is the coupling condenser from theoscillator anode 20 to the oscillator tank coil II. The plate 2l' oftube I0' has the FM oscillations developed in its circuit, because ofthe electron coupling between the latter and the oscillatorseccomponents from being presentrin the modulation signal outputcircuit. In the case of record reproduction the modulating signal is ofaudio frequency. Resistor 36' is the detector load resistor. The audiooutput energy may be amplied, and then reproduced.

The systems shown in Figs. and 5a owe their improvement to the genericbalanced action which forms the basis of my presently-disclosed method,and which makes it possible to keep the mass of the vibratory system aslow as possible. In a pickup it is most important to keep the vibratorysystem mass as low as possible. This raises the frequency of the peak,and reduces the mechanical impedance thereby permitting a reduction ofneedle pressure and theaccompanyingl reduction of surface noise. Thebalanced pickup employed in the presentl invention has the stylussupporting wire act as the center plate I5 of the push-pull condenserwhich actuates the balanced FM network. This results in a reduction ofthe pick-up mass to a minimum, since nothing else would be mechanicallycoupled to the stylus except its own support. Ihis supporting wireitself is very small in diameter, so that no extra mass would be addedin transforming the mechanical vibrations into electrical frequencies.

Figs. 6, '1, 8 and 9 show the constructional details of the novelbalanced pickup device. Considering Fig. 6, a sapphire stylus Bn ismounted at the forward end of a steel wire 6I. The wire 6 I, which isthe supporting wire mentioned above, is bent into the shape shown indetail in Fig. 8. The upper end of the supporting wire 6I is mounted ona small brass block 62. This assembly of the stylus, supporting wire andbrass block is shown in detail in Fig. 8. The stylus supporting wire isground flat along its vertical plane on both sides and along its lengthto about 0.008 inch, and forms the center grounded plate electrode I5 ofthe balanced'condenser I4-I5-I6.

In Fig. 9 there is shown the cross-sectional appearance of thesupporting wire 6I. Merely by way of illustration, it is pointed outthat the vertical section of the supporting wire 6| in Fig. 8 may have adimension ofv0.09 inch; the distance between the bend in the wire andthe sapphire point may be equal to 0.13 inch, while the sapphire pointmay have a radius of 0.0028 inch. In Fig. 9 the long dimension may be0.014 inch, while the short dimension may be 0.008 inch. A fixedcondenser plate has been removed in Fig. 6 from in front of tbe'supporting wire 6I.

The assembly which has been described to this point isthen mounted on thfront of a Bakelite block B3 by means of a screw member 64. Two xedcondenser plates 65 and 66, both are shown in Fg. 7 while only plate 65is shown in Fig. 6, are then mounted in the Bakelite block so as to beparallel to, and on each side of, the stylus support wire 6I. Each ofthe xed condenser plates is preferably spaced a distance of about 0.002in'ch. or 0.003 inch, from the stylus support wire 6I. These two plates,65 and 66, form the two outside plates of the balanced condenser,.

and correspond to plates I4 and I6 of Fig.' 5.

The addition of a small Viscoloid strip 61,

which is mounted between the stylus 60 and the mounting screw 64,completes the pickup assemtorsional resonance of the stylus supportwire. 'I'he pickup assembly is secured to the tone arm 10 by means ofthe two spaced spring plates 80 and thatl permit it to be bent in avertical.. t

direction. The purpose of this so-called compliance is to prevent damageto the pickup head when it is accidentally dropped on a record. 'I'hearm 10 consists of a conventional U-shaped channel, but has a supportingshield 9i running lengthwise and forming two separate channels.

The two leads from the pickup head, which may be regarded as radiofrequency transmission lines,

are placed inside of the two corresponding chanconnected to thediscriminator coil. The stylus support wire 6I is connected to the armproper, and, in turn, to the chassis which is grounded.

In Fig. 10 there is shown a response curve secured with the presentinvention. The needle pressure required for proper trackingon theloudest passages was found to be only 0.2 ounce. In actual testing,however, and for a wider safety margin 0.33 ounce was used. Theresonance frequency was found to be at 21,000 cycles, having a 10decibel dip at 16,000 cycles due to the stylus support Wire developing anode at its middle. The rest of the response curve was fiat within onedecibel to 65 cycles as shown by curve .A in Fig. 10. Curve B in Fig. 10shows the response when the "Viscoloid damping member was used. Thedamping reduced the peak and dip, and also prevented the torsionalresonance of the wire. 'I'he response in the useful frequency range,however, was unaltered, but otherwise stabilized the performance of thepickup. Curve C shows the response when a so-called "Orthacousticnetwork is inserted in the audio amplifier network of Fig. 5 or Fig. 5a.On actual hiss measurements it was found that hiss was very low, andabout 7 decibels lower than with other types of commercial pickups whichhave been considered very satisfactory heretofore. p

The present balanced FM circuit has other anplications. For example, itmay be used in connection with a record cutter.. Again, the presentinvention may be employed in connection with a push-pull form ofcondenser microphone. In that case the diaphragm would be the centerplate of the push-pull condenser with the usual plate at the back of it.In front of the diaphragm would be placed another plate, but perforatedwith conically-shaped holes to allow the sound to pass through anddeflect the diaphragm. The remainder of the system would be very similarto that shown in either of Figs. l, 5 or 5a. In general, the presentinvention is applicable to measurement devices where linearity and smallpickup area are essential. Such uses would include pressure devices,liquid height measuring devices, elongation measurement, vibrationmeasurement and the like. .When the pickup is a microphone, the systemcould be used in hearingaid apparatus, public address systems, 'binauralsystems, and the like.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular circuitsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:

1. In combination, a tube provided with an oscillator section and anoutput electrode, means connected to said oscillator section to provideangle modulated oscillations which have a predetermined mean frequency,a resonant circuit coupled to said output electrode, means providingcoupling between said oscillator section and said output electrode fordeveloping said angle modulated oscillations in the circuit connected tosaid output electrode, said resonant circuit being tuned to a frequencysufficiently different from said mean frequency to permit the resonantcircuit to function as a discriminator circuit, means mechanicallyresponsive to operation of said firstnamed means for providing anglemodulated oscillations at the discriminator circuit independently ofsaid coupling, and means coupled to the discriminator resonant circuitfor rectifying resultant amplitude modulated wave energy appearingacross the discriminator circuit.

2. In combination with a capacity pickup device for phonograph records,an oscillator having a resonant circuit tuned to a predetermined meanfrequency, a detector having a resonant input circuit coupled to saidoscillator, said detector input circuit being tuned to a peak frequencywhich differs from the said mean frequency by a predetermined frequencyvalue whereby the detector input circuit functions as a discriminatorcircuit, and said capacity pickup device consisting of a rst condenserconnected across said oscillator resonant circuit and a second condenserconnected across said discriminator circuit, said two condensers havinga common mobile element for concurrently but oppositely varying thecapacities of the two condensers. l

3. A method which includes producing oscillations of a predeterminedfrequency, transmitting said oscillations to a network whose frequencydiffers from said oscillation frequency by a predetermined frequencyvalue, deviating said oscillations in frequency relative to saidfirst-mentioned predetermined frequency in accordance with signals,concurrently deviating the frequency of said network with respect to itsfrequency, maintaining said frequency deviations in 180 degree phaserelationship, and utilizing the resultant of the two frequencydeviations,

. 4. In combination with an oscillator of a predetermined high frequencyprovided with a resonant circuit, a source of modulation signals, adiscriminator circuit coupled to the oscillator, said discriminatorcircuit having a resonance curve Whose peak frequency is of apredetermined value different from the oscillator frequency, saidoscillator frequency being located at a substantially linear portion ofa ank of the resonance curve, a first capacity device provided acrossthe oscillator resonant circuit, a second capacity device providedacross said discriminator circuit, and said source of modulation signalsbeing connected to said two capacity devices for varying the magnitudesthereof in opposite directions thereby to provide balanced frequencymodulation.

5. In combination with a transducer device, an

-mean frequency by a predetermined frequency value whereby the detectorinput circuit functions as a discriminator circuit, and said transducerdevice consisting of a rst frequency changing component of saidoscillator resonant circuit and a second frequency changing com- Ponentof said discriminator circuit, said two components having a commonmobile element for concurrently but oppositely varying the values ofsaid components.

6. In a frequency modulation system, a pair of tuned circuits normallytuned to different frequencies separated by a predetermined frequencyvalue, and means for varying the frequencies of said circuits,respectively, simultaneously in opposite directions in accordance with amodulation signal.

7. In a frequency modulation system, a pair of tuned circuits, saidtuned circuits being normally tuned to different frequencies spacedapart by a predetermined frequency value, and means for varying thefrequencies of said circuits, respectively, simultaneously in oppositedirections in accordance with a modulation signal, said varying meansbeing a transducer.

8. In a frequency modulation system, an oscillator including a rst tunedcircuit, a discriminator coupled to said oscillator andincluding asecond tuned circuit, means for varying the frequencies of saidcircuits, respectively, in opposed phase relation in accordance with asignal.

9. In a frequency modulation system, an oscillator including a firsttuned circuit, a discriminator coupled to said oscillator and includinga second tuned circuit, means for varying the frequencies of saidcircuits, respectively, in opposed phase relation in accordance with asignal, said means being a transducer comprising a frequency changingcomponent for each of said circuits, and a vibratile element common tosaid components.

10. In a frequency modulation system, an oscillator including a rsttuned circuit, a discriminator coupled to said oscillator and includinga second tuned circuit, a transducer having a pair of frequency changingelements for said circuits, respectively, disposed in a plane, avibratile element commento said pair of elements and supported forlateral movement relative to said pair of elements for varying thefrequency of said circuits in opposed phase relation, and said vibratileelement being constructed and arranged to that transverse movement ofsaid element normal to said plane effectively cancels out the effect ofsaid transverse movement.

11. In combination, a tube provided with an oscillator section and apositive output electrode, transducer means connected to said oscillatorsection to provide frequency modulated oscillations which have apredetermined mean frequency, a resonant circuit coupled to said outputelectrode, electron coupling between said oscillator section and saidoutput electrode developing said frequency modulated oscillations in thecircuit connected to said output electrode, said resonant circuit beingtuned to a frequency sumciently different from said mean frequency topermit the resonant circuit to function as a discriminator circuit, asecond transducer means responsive to operation of said first-namedmeans for providing frequency modulated oscillations at thediscriminator circuit independently of said electron coupling, and adiode, provided byl electrodes of said -tube, coupled to thediscriminator resonant circuit for rectifying the wave energy appearingacross the discriminator circuit whichis amplitude modulated.

12. In combination with a pickup device for phonograph records, anoscillator having a resonant circuit tuned to a predetermined meanfrequency, a detector having a resonant input circuit coupled to saidoscillator, said detect@ input circuit bein-g tuned to a peak frequencywhich differs from the said mean frequency by a predetermined frequencyvalue whereby the detector input circuit functions as a discriminatorcircuit, and said pickup device consisting of a rst reactance connectedacross said oscillator resonant circuit and a second reactance connectedacross said discriminator circuit, said two reactances having a commonmobile element for concurrently, but oppositely, varying the magnitudesof the two reactances.

13. A method which includes producing oscillations of a predeterminedfrequency. transmitting said oscillations to a network whose frequencydiffers from said oscillation frequency by a predetermined frequencyvalue, deviating said oscillations in frequency relative to saidpredetermined frequency in accordance with modulating signals,concurrently deviating the frequency of said network with respect to itsfrequency, maintaining said frequency deviations in 180 degree phaserelationship, subjecting the resultant of the two frequency deviationsto frequency discrimination, and rectifying the resultingamplitude-variable energy.

14. In combination with an oscillator of a predetermined highlfrequencyprovided with a resonant circuit, a source of audio modulation signals,a discriminator circuit coupledt the oscillator,

Asaid discriminator circuit having a resonance m8 BADMAIEFF.

